1. Field of the Invention
The invention relates to a ribbon for a ribbon microphone which can reduce deterioration of a frequency response caused by a partial resonance of the ribbon, a manufacturing method of the same, and the ribbon microphone.
2. Description of the Related Art
A ribbon microphone is mainly comprised of magnets for forming a magnetic field and a ribbon. The magnets are supported on both sides of a frame so as to sandwich the ribbon and the magnetic field is formed between the magnets on both sides. The ribbon is arranged in the magnetic field in a state where a proper tension has been applied to the ribbon and both edge portions in the length direction have been pressed. When the ribbon receives a sound wave and vibrates in the magnetic field, a current according to the sound wave flows in the ribbon and the sound wave is converted into an electric signal. Hitherto, aluminum foil has widely been used as a material of the ribbon. Aluminum is suitable as a ribbon of the ribbon microphone because its conductivity is better and its specific gravity is lower as compared with those of other metal materials.
FIG. 5 shows an example of a general ribbon microphone in the related art. FIGS. 6 and 7 show a ribbon microphone unit built in the ribbon microphone. In FIG. 5, a box of a ribbon microphone 1 is comprised of a cylindrical base 6 and a microphone case 2 coupled with an upper edge of the base 6. In this box, a ribbon microphone unit 3 is assembled to a proper supporting member fixed to the base 6. The microphone unit 3 is covered with the microphone case 2. A lower edge of the base 6 is a connector 16 for connecting a microphone cable to guide an output signal of the microphone to an external circuit.
As also shown in FIGS. 6 and 7, the ribbon microphone unit 3 has a frame 7 formed in a rectangular frame die which is long in the vertical direction. A pair of permanent magnets 4 are fixed to inside surfaces of the frame 7 on both sides along the long-side direction at a predetermined interval between the permanent magnets 4. The permanent magnets 4 have been polarized in the width direction (right/left direction in FIGS. 5 and 6). The polarizing directions of the pair of permanent magnets 4 are the same direction and a parallel magnetic field is formed between the permanent magnets 4.
A ribbon 5 serving as a diaphragm and a conductive material is arranged in the parallel magnetic field. The ribbon 5 is an elongated belt-shaped member. Both edge portions in the length direction of the ribbon 5 are fixed to electrode lead-out portions 18 provided in both edge portions in the length direction of the frame 7. The electrode lead-out portions 18 are insulated from the frame 7. Tightening members 8 for sandwiching the edge portions in the length direction of the ribbon 5 are arranged in the electrode lead-out portions 18. The both edge portions in the length direction of the ribbon 5 is sandwiched by the tightening members 8 forming a pair respectively, so that the ribbon 5 and the tightening members 8 are made conductive and the ribbon 5 is held in a state where the moderate tension has been applied to the ribbon 5. In the both edge portions of the ribbon 5, portions 51 other than the portions locating in the electrode lead-out portions 18 are alternately bent at a predetermined interval, so that they are formed in a corrugated shape. The directions of the lines formed by the bending, that is, the directions of the lines drawn by a top portion and a bottom of the waveform coincide with the width direction of the ribbon 5 and those lines are formed at a predetermined interval. The portions formed in the corrugated shape in the both edge portions of the ribbon 5 are assumed to be the corrugated edge portions 51 hereinbelow. Terminal plates 9 are overlaid onto the tightening members 8 of the both edge portions. The terminal plates 9 are electrically conductive to each of the edge portions of the ribbon 5 through the tightening members 8, thereby allowing a signal from the ribbon microphone unit 3 to be outputted from each terminal plate 9. An intermediate portion of the ribbon 5 sandwiched by the corrugated edge portions 51 on both sides is a corrugated intermediate portion 52 in which a top and a bottom of a triangular wave are formed along the line in the length direction which perpendicularly crosses the directions of the lines drawn by the top and the bottom of the waveform of each of the corrugated edge portions 51, that is, the length direction of the ribbon 5.
The ribbon 5 receives the sound wave and vibrates according to the sound wave. The vibrating direction is a direction which traverses a magnetic flux between the permanent magnets 4. When the ribbon 5 made of the conductive material traverses the magnetic flux, the ribbon 5 generates an electric power and an electric signal is generated between the both edges in the length direction of the ribbon 5, accordingly, between the electrode lead-out portions 18. Since the electric signal is a signal having a frequency and an amplitude corresponding to a frequency and an amplitude of the ribbon 5, the sound wave which is applied to the ribbon 5 is converted into an electric signal corresponding to the sound wave. Since the ribbon microphone is an inertia control type microphone, it is necessary to set a resonance frequency of the ribbon 5 to a frequency which is equal to or lower than a low-band frequency of the sound wave to be collected, in other words, a frequency lower than the lowest frequency of a frequency band of the sound which can be collected. For this purpose, the tension of the ribbon 5 is set to an extremely low. As mentioned above, by bending the ribbon 5 into the corrugated shape, the low tension is realized.
In the example of the ribbon microphone described above, the intermediate portion in the length direction of the ribbon 5 sandwiched by the corrugated edge portions 51 is the corrugated intermediate portion 52 in which the top portion and the bottom of the triangular wave are formed along the line in the length direction of the ribbon 5. Ribbons of many ribbon microphones in the related art are formed in such a manner that the directions of the lines drawn by the top portion and the bottom of the triangular wave coincide with the width direction of the ribbon in the whole length direction. In other words, they are formed so that the triangular wave progresses in the whole length direction. To manufacture the ribbons of such a form, a pair of two spur gears in which teeth each constructed by a mountain and a valley of a triangular wave are formed around an outer periphery at a predetermined interval have been used in the related art. As a pair of spur gears, the gears of exactly the same specifications as well as a pitch of teeth and a radius are used. The manufacturer puts the pair of spur gears onto a flat and smooth plate such as a glass plate, sandwiches the aluminum foil as a material of the ribbon 5 by the pair of spur gears, rotates the pair of spur gears in the opposite directions while mutually pressing, and feeds out the material. In this manner, the form of the teeth of the pair of spur gears is transferred to the material and the ribbon of the waveform as mentioned above is formed.
According to the ribbons which are manufactured as mentioned above, since the material is the metal foil made of aluminum or the like, resonance sharpness of the mechanical resonance is high. That is, a peak due to the resonance appears sharply. Therefore, a problem exists in that the resonance appears in the frequency response and the sound wave cannot be converted into the electric signal with fidelity. FIG. 8 illustrates a state of occurrence of a partial resonance in the ribbon formed in such a manner that the directions of the lines drawn by the top and the bottom of the triangular wave coincide with the width direction in the whole length direction. The partial resonance occurs in the portions corresponding to one period and half period of the triangular wave as shown by being surrounded by two ellipses of broken lines in FIG. 8. This is because when paying attention to the portion corresponding to the half period, since this portion has a flat surface, its rigidity is low and the resonance is liable to occur. FIG. 9 shows frequency response characteristics of the microphone using such a ribbon. An axis of abscissa indicates a frequency and an axis of ordinate indicates an output signal level (unit: dBV). Two lines shown on the upper side of this graph indicate measurement results obtained by measuring at the front and rear of the microphone. Two lines shown on the lower side of this graph indicate measurement results obtained by measuring on the right and left of the microphone. Since the characteristics at the front and rear are important in the ribbon microphone, when paying attention to the characteristics at the front and rear, sharp peaks appear periodically. They are caused due to the foregoing partial resonance.
The partial resonance as mentioned above appears clearly in the ribbon formed in such a manner that the directions of the lines drawn by the top and the bottom of the triangular wave coincide with the width direction in the whole length direction. However, the partial resonance also appears in the ribbon formed with the corrugated intermediate portion 52 in which the top and the bottom of the triangular wave are formed along the line in the length direction of the ribbon as shown in FIGS. 5 to 7. This is because the portion corresponding to the half period of the triangular wave has also a flat surface and its rigidity is low.
As a countermeasure for improving such a problem in the related art, it is considered that the invention disclosed in Patent Document 1 (JP-A-2007-49324) which has been proposed by the present inventor before is adopted. The invention disclosed in Patent Document 1 relates to a manufacturing method of a ribbon for a ribbon microphone constructed in such a manner that a material of the ribbon is put onto the surface of a transfer mold formed in the same shape as that of the ribbon to be molded and is temporarily fixed and a roller having an elastic member on an outer circumferential surface is rolled while pressing the elastic member to the material of the ribbon, thereby transferring the surface shape of the transfer mold onto the material of the ribbon. According to Patent Document 1, by subjecting embossing or satin finish to the surface of the intermediate portion in the length direction of the transfer mold, the embossing finished or satin finished shape is transferred to the intermediate portion in the length direction of the ribbon, so that a rigidity of the length directional intermediate portion is high to a certain extent.
In Patent Document 1, there is also such a disclosure that it is preferable to form the elastic member of the roller by electrostatic flocking.
According to the invention disclosed in Patent Document 1, the rigidity of the length directional intermediate portion of the ribbon can be increased to a certain extent. However, even if the shape such as embossing finished or satin finished shape is formed, the rigidity of the ribbon cannot be sufficiently increased and the problem of the partial resonance cannot be solved.